1. Field of the Invention
The present invention relates in general to capacitor charging current measurement methods and, more particularly, to a charging current measurement method adaptable for use in measuring the insulation resistance of capacitors.
2. Description of the Prior Art
Generally, for purposes of reliability testing, there is known a method for measuring the insulation resistance of a capacitor by applying a direct current (DC) measurement voltage to the capacitor and by measuring a leakage current (charging current) of such capacitor after full charge-up thereof. Obviously, normal or acceptable products are lower in leakage current.
A well known charging current measurement method is defined by JIS-C5102. This scheme requires that current value measurement be done under the condition that a capacitor under measurement is fully charged up, which in turn requires a measurement time of approximately 60 seconds to elapse. However, as it is becoming more strictly demanded to reduce manufacturing costs while increasing reliability of electronic equipment, a further increase in productivity and quality is also demanded for electronic components such as capacitors for use with such equipment; in view of this, the prior known measurement method which needs a substantial length of measurement time duration per capacitor is no longer capable of fully responding to such demands.
Another method has been proposed in which a charging current value is measured at a plurality of times within a shortened time period immediately after initiation of application of a voltage to a capacitor, thereby predicting, based on the resultant multiple current measurement values a current value which will occur after of a predefined time elapses (Japanese Patent Publication No. 5-78790). This method is designed to measure current values I.sub.0, I.sub.1, I.sub.2 flowing through a capacitor at three different time points with constant time intervals therebetween, and to calculate, based on these three current measurement values, the current value I.sub.x after a predefined length of time elapses, by use of an equation as follows: EQU I.sub.x =(I.sub.1.sup.2 -I.sub.0 .multidot.I.sub.2)/(2I.sub.1 -I.sub.2 -I.sub.0).
Use of this method enables computation or calculation of a result prior to the time that the first-mentioned method would take to reach a sufficient charge-up state, which might advantageously make it possible to measure an intended insulation resistance within a shortened time period. However, the above equation is based on the assumption that the equivalent circuit of a capacitor is as shown in FIG. 1. Therefore, an accurate insulation resistance will no longer be definable in the case of capacitors having a dielectric polarization component, such as ceramic capacitors.
More specifically, as shown in FIG. 1, a simplified capacitor equivalent circuit is comprised of a capacitance C.sub.0, internal resistance r and insulation resistance R.sub.0. However, an equivalent circuit of a real capacitor also includes a dielectric polarization component D, as shown in FIG. 2. It may be considered that during the initial chargeup period (10 milliseconds after the initiation of the charging operation, for example), the effects of the capacitance C.sub.0, internal resistance r and insulation resistance R.sub.0 appear most strongly; thereafter, however, the dielectric polarization component D controls the chargeup characteristics. Accordingly, with the prior art prediction method ignoring the presence of such dielectric polarization component D, it remains impossible or difficult to accurately predict the charging current at termination of the chargeup period (one minute later, for example).